Most nonspecialized home and industrial electrical apparatus in the United States is driven either by 110 volt or 220 volt AC, 60 cycle per second power. Many applications require AC electrical power with either a frequency different from 60 cycles, or a voltage other than 110 or 220 volts AC. For example, stage lighting systems often require low voltages, but high power. In generating AC electrical power with a frequency or voltage magnitude different from the standard line power, it is conventional to use a transformer for isolation purposes, as well as to provide an increase or decrease in the magnitude of the voltage. However, when used in connection with 60 cycle power, a rather large iron-core transformer must generally be used, thus presenting space and weight problems in such systems as stage lighting apparatus. In addition, when a highly regulated AC output voltage is required, additional regulation circuitry must be used.
U.S. Pat. Nos. 3,237,082 and 3,930,193 disclose converters with regulated output parameters. Disclosed in U.S. Pat. No. 3,360,710 is a converter for providing an output frequency which is adjustable, and which is independent of the input AC line voltage frequency. Provisions are also disclosed in the latter-mentioned patent for regulating the output AC voltage of a converter to a desired magnitude.
In each of the above-noted patents, the input AC voltage to the converter is rectified and filtered to produce a DC voltage, which is converted into the desired AC frequency and voltage. In the prior U.S. Pat. Nos. 3,237,082 and 3,930,193, rectifier diodes and a parallel capacitor provide a filtered DC voltage which is switchably controlled by a series pass transistor for feeding the DC voltage to an output AC generating section. In U.S. Pat. No. 3,360,710, silicon controlled rectifier (SCR) devices provide current to the DC filter section of the converter.
In the converters disclosed in U.S. Pat. Nos. 3,237,082 and 3,930,193, the input filter capacitors of the DC section are charged according to the amplitude of the rectified input AC voltage. The output voltage of these power supplies is controlled by circuits in the output section of the supplies. With regard to U.S. Pat. No. 3,360,710, the capacitor voltage of the DC filter section is controlled by the firing of the rectifying SCR devices. The shortcoming with using SCR devices is that once turned on, the devices remain on until the bulk current therethrough is reduced to zero. Therefore, in order to charge the filter capacitors to a predetermined voltage, the SCR must be triggered on at some point in the AC cycle and turned off when the AC current passes through a zero current point. The sudden conduction of the SCR causes an in-rush current, a large di/dt, to the capacitors which can cause long term damage to the electrolytic action of the capacitors. Conventional converters require large inductors to limit the rate of change of current (di/dt) in SCR systems. Such systems that have a rapid turn off further generate radio frequency interference and electromagnetic interference (RFI/EMI). The interference generated by converters of this type may exceed the allowable standards.
From the foregoing, it may be seen that a need has arisen for a converter which has neither large input transformers nor SCR devices, and thus can be housed in small enclosures such as stage lamp facilities. There is an associated need for a reliable power converter where the charging currents for the filter capacitors are synchronized with the input AC zero crossings, such that the capacitor charging rate is similar to the AC current as it increases from a zero crossing.